1. Field of the Invention
The present invention relates to a heating resistance element, a thermal head and a printer using the same, and a method of manufacturing a heating resistance element.
2. Related Background Art
A heating resistance element is used in, for example, a thermal head of a thermal printer. In a typical structure, a thermal storage layer made of glass or the like is provided on a substrate made of alumina ceramic or the like, and a plurality of heating resistors are provided on the thermal storage layer.
Here, a thermal printer is a generic name for a thermal transfer printer for transferring ink heated and fused by a thermal head onto an object to be printed, a direct thermal printer for directly forming an image on thermal paper by a thermal head, and the like.
In a thermal printer, by making heating resistors of a thermal head to selectively generate heat, and by applying heat to an object to be heated such as an ink ribbon or thermal paper at a desired position, ink is fused and transferred onto an object to be printed in a desired pattern, or a desired pattern is formed on thermal paper.
As equipment using such the heating resistance element, in recent years, power saving products capable of being driven by a battery and mainly used as small sized and lightweight portable equipment are widely in use. Further, recently, due to energy circumstances in view of saving the environment or the like, power saving such as no power consumption in a dormant state is actively promoted even for stationary electronic equipment using no battery. Also, it is essential to increase energy efficiency.
It is said that, with a conventional heating resistance element, most heat generated by heating resistors does not contribute to printing or the like which is a target of a heating process, and that the heat is transferred to a substrate side through a material forming the heating resistance element or a thermal storage layer.
Therefore, attempts are made to attain power saving of the heating resistance element by preventing the heat generated by the heating resistors from being transferred to the substrate as much as possible, and by making effective use of the heat for a heating process such as printing (that is, by increasing the heating efficiency).
Further, when a thermal head continuously performs print output, since heat is continuously transferred to the substrate, heat radiation from the substrate cannot keep up with the heat transfer, and the whole thermal head is brought up to a considerably high temperature. Because this temperature rise is a cause of deterioration of print quality, in order to materialize high quality continuous printing, it is necessary to increase the heating efficiency of the thermal head.
As a thermal head with an increased heating efficiency, one structured as disclosed in Japanese Patent Application Laid-open No. Hei 6-166197, for example, has been devised. This thermal head has a structure in which a plurality of heating resistors are provided with intervals therebetween on a surface of an insulating substrate composed of an insulating substrate body and an underglaze layer formed on a surface of the insulating substrate body, and in which wiring for supplying electric power to these heating resistors is provided. Attempts are made to make the band-like hollow portion function as a heat insulating layer having low thermal conductivity, and to decrease the amount of heat transferred from the heating resistors to the insulating substrate side, and thus, to improve the heating efficiency, by providing a band-like hollow portion extending along a direction of arrangement of the heating resistors at a midpoint in a thickness direction of the underglaze layer.
The band-like hollow portion is formed in the underglaze layer by embedding a band-like cellulosic resin when the underglaze layer is being formed, and by vaporizing the cellulosic resin in a baking process.
However, the thermal head disclosed in Japanese Patent Application Laid-open No. Hei 6-166197 has the following problems.
First, although a provision of the hollow portion under the heating resistors has a thermally insulating effect in a direction of the insulating substrate body, because the hollow portion is formed at the midpoint in the thickness direction, it is necessary for the underglaze layer itself to be formed relatively thick. Therefore, the amount of heat transferred to the underglaze layer accumulates in the underglaze layer. Accordingly, since the amount of heat transferred to a surface side of the heating resistors is small, the heating efficiency is low.
Second, dimensional precision of the resin material to be vaporized for forming the hollow portion is low, so a precisely shaped hollow portion cannot be formed. Therefore, because the hollow portion is formed to be band-like across the plurality of heating resistors along the direction of arrangement of the plurality of heating resistors, the strength of the underglaze layer at the positions of the heating resistors is low, and thus, the hollow portion is liable to crush due to pressure applied to the heating resistors in printing. In particular, because a drum, which sandwiches printing paper with the heating resistors, is disposed along the direction of arrangement of the heating resistors, there is a fear that the underglaze layer cracks along the direction of arrangement of the heating resistors.
Third, in a conventional method in which the hollow portion is provided at the midpoint in the thickness direction of the underglaze layer, a vaporization component layer made of a cellulosic resin is printed on a surface of an underglaze lower layer so as to be band-like and is then dried. After that, an underglaze surface layer forming paste made of a same insulating material as that of the underglaze lower layer is formed on a surface and is then dried. Further, by baking the thus laminated insulating material at about 1300° C., the vaporization component layer is vaporized. Therefore, complicated processes are necessary for providing the hollow portion under the heating resistors, and requires much time in manufacture.